Reaction: S-adenosyl-L-methionine + 3,4-dihydroxy-5-all-trans-polyprenylbenzoate = S-adenosyl-L-homocysteine + 3-methoxy-4-hydroxy-5-all-trans-polyprenylbenzoate
For diagram of reaction click here.
Other name(s): 3,4-dihydroxy-5-hexaprenylbenzoate methyltransferase; dihydroxyhexaprenylbenzoate methyltransferase; COQ3 (gene name); Coq3 O-methyltransferase; DHHB O-methyltransferase
Systematic name: S-adenosyl-L-methionine:3,4-dihydroxy-5-all-trans-polyprenylbenzoate 3-O-methyltransferase
Comments: This enzyme is involved in ubiquinone biosynthesis. Ubiquinones from different organisms have a different number of prenyl units (for example, ubiquinone-6 in Saccharomyces, ubiquinone-9 in rat and ubiquinone-10 in human), and thus the natural substrate for the enzymes from different organisms has a different number of prenyl units. However, the enzyme usually shows a low degree of specificity regarding the number of prenyl units. For example, the human COQ3 enzyme can restore biosynthesis of ubiquinone-6 in coq3 deletion mutants of yeast . The enzymes from yeast and rat also catalyse the methylation of 3-demethylubiquinol-6 and 3-demethylubiquinol-9, respectively  (this activity is classified as EC 18.104.22.168, 3-demethylubiquinol 3-O-methyltransferase).
Links to other databases: BRENDA, EXPASY, KEGG, Metacyc, CAS registry number: 139569-31-6
1. Clarke, C.F., Williams, W., Teruya, J.H. Ubiquinone biosynthesis in Saccharomyces cerevisiae. Isolation and sequence of COQ3, the 3,4-dihydroxy-5-hexaprenylbenzoate methyltransferase gene. J. Biol. Chem. 266 (1991) 16636-16641. [PMID: 1885593]
2. Poon, W.W., Barkovich, R.J., Hsu, A.Y., Frankel, A., Lee, P.T., Shepherd, J.N., Myles, D.C. and Clarke, C.F. Yeast and rat Coq3 and Escherichia coli UbiG polypeptides catalyze both O-methyltransferase steps in coenzyme Q biosynthesis. J. Biol. Chem. 274 (1999) 21665-21672. [PMID: 10419476]
3. Jonassen, T. and Clarke, C.F. Isolation and functional expression of human COQ3, a gene encoding a methyltransferase required for ubiquinone biosynthesis. J. Biol. Chem. 275 (2000) 12381-12387. [PMID: 10777520]
4. Xing, L., Zhu, Y., Fang, P., Wang, J., Zeng, F., Li, X., Teng, M. and Li, X. Crystallization and preliminary crystallographic studies of UbiG, an O-methyltransferase from Escherichia coli. Acta Crystallogr. Sect. F Struct. Biol. Cryst. Commun. 67 (2011) 727-729. [PMID: 21636923]